March 2  6, 2009
In NMR spectroscopy, the collective measurement is weakly invasive and its backaction is called radiation damping. The aim of this talk is to provide a controltheoretical analysis of the problem of suppressing this radiation damping. It is shown that the two feedback schemes commonly used in the NMR practice correspond one to a high gain oputput feedback for the simple case of maintaining the spin 1/2 in its inverted state, and the second to a 2degree of freedom control design with a prefeedback that exactly cancels the radiation damping field. A general high gain feedback stabilization design not requiring the knowledge of the radiation damping time constant is also investigated.
Interaction of ultrashort intense laser pulses with molecular
media leads to
highly nonlinear nonperturbative effects which can only be
treated by large
scale computation on massively parallel computers. Single
molecule response to
such pulses leads to Molecular High Order Harmonic Generation,
MHOHG, (1), from
which one can synthesize new "attosecond" pulses necessary to
control electron
dynamics at the natural time scale of the electron, the
attoseocond (10**18 s),
(2).The single molecular response can be obtained from high
level quantum
TimeDependent Schrödinger,TDSE, simulations. The collective
macroscopic
response of a molecular medium requires solving many TDSE,s
(>10**5)coupled to
the classical laser (photon) Maxwell equations (3). We will
present the
numerical methods necessary to achieve this goal, especially
the problem of
transparent and artificial boundary condition techniques in
view of the
different time scales, photon vs electron. Results will be
shown for attosecond
pulse generation and pulse filamentation in an aligned
molecular medium, the
one electron H2+ system(4).
(1).A D Bandrauk et al,"Molecular Harmonic Generation," in
Progress in Ultrafast
Intense Laser Science, vol III, edit K. Yamanouchi (Springer
V, NY,2008), chapt 9.
(2).A D Bandrauk,F Krausz, A Starace, "Focus on Attosecond
Physics," New J
Phys, 10, 025004(2008).
(3).E Lorin,S Chelkowski, A D Bandrauk,"MaxwellSchrödinger
Equations for
Nonlinear Laser Propagation in Molecular Media," Comput.
Phys. Commun. 177, 908 (2007).
(4).E Lorin,S Chelkowski,A D bandrauk,"Attosecond Pulse
Generation for Aligned
Molecules," New J Phys, 10, 025033(2008).
In this talk we study the dissipative dynamics arising from coupling
to an infinite field in both the classical and quantum context. In
particular we study dissipative dynamics generalizing the classical
Lamb model. We apply this to the study of dissipation arising in
certain controlled quantum systems and also study a model
which allows us to quantize certain nonholonomic systems. In the latter
case we consider nonholonomic constraints as arising from the limit
of a frictional force and then implement the force by an external field
which we then quantize. Other methods of quantizing nonholonomic systems will
also be discussed.
Realistic quantum mechanical systems are influenced through
the coupling to an environment containing a large number of
mostly uncontrollable degrees of freedom. This unavoidable
interaction of an open quantum systems with its environment
leads to the mechanisms of dissipation and damping, and to
a strong and often rapid loss of quantum coherence. The
talk begins with a brief introduction into the standard
theory of quantum mechanical relaxation which is based on
the Markov approximation and on the concepts of completely
positive dynamical semigroups and of quantum master
equations in Lindblad form. Many examples for this
approach are known from quantum optics, decoherence theory,
quantum Brownian motion and quantum measurement and control
theory. However, strong couplings or interactions with
lowtemperature reservoirs generally lead to large
systemenvironment correlations which result in long memory
times and in a failure of the Markov approximation. To
describe the basic features of the nonMarkovian quantum
dynamics of open systems we develop several new methods as,
for example, the technique of correlated projection
superoperators [1] and the concept of quantum semiMarkov
processes [2]. A number of examples and applications to
structured and finite reservoirs [3], to electron spin
dynamics in quantum dots [4], and to the problem of
quantum transport in nanostructures [5] will be discussed.
[1] H. P. Breuer, Phys. Rev. A 75, 022103 (2007).
[2] H. P. Breuer and B. Vacchini, Phys. Rev. Lett. 101,
140402 (2008).
[3] H. P. Breuer, J. Gemmer and M. Michel, Phys. Rev. E73,
016139 (2006).
[4] E. Ferraro, H. P. Breuer, A. Napoli, M. A. Jivulescu,
and A. Messina, Phys. Rev. B78, 064309 (2008).
[5] R. Steinigeweg, H. P. Breuer and J. Gemmer, Phys. Rev.
Lett. 99, 150601 (2007).
We present a recently developed mixed quantumclassical method which accounts
for the evolution of a quantum subsystem coupled to a nonequilibrium
environment (solvent) described in an extended hydrodynamic setting [1].
Starting from a hybrid quantumclassical phasespace distribution, coupled
equations for the quantumclassical local density and momentum density are
derived which feature the characteristic populationcoherence coupling of the
nonadiabatic quantum evolution. A generalized free energy functional is
introduced, which is similar to the functionals used in dynamical density
functional theory (DDFT) methods [2] but is adapted to the quantumclassical
setting. The relevant functionals involve twoparticle (or, more generally,
nparticle) correlation functions that are constructed from statespecific
microscopic solutesolvent interactions. A microscopic Marcustype functional
for polar solvation is considered as a special case. The present formulation
is particularly appropriate to describe ultrafast solvation dynamics coupled
with charge transfer, for example in photochemical charge transfer processes.
By the explicit consideration of quantum coherence, the details of population
transfer and its susceptibility to decoherence effects, become amenable to
direct investigation. First numerical examples are presented [3] and the
extension of the formalism beyond the free energy functional formulation are
addressed, in particular in view of including nonequilibrium solvent
correlations.
[1] I. Burghardt and B. Bagchi, Chem. Phys. 329, 343 (2006).
[2] B. Bagchi and A. Chandra, Adv. Chem. Phys. LXXX, 1 (1991);
U. Marini Bettolo Marconi and P. Tarazona, J. Chem. Phys. 110, 8032 (1999);
A. J. Archer and R. Evans, J. Chem. Phys. 121, 4246 (2004).
[3] P. Ramanathan, S. Parry, S.L. Zhao, K. H. Hughes, and I. Burghardt,
to be submitted.
In this talk we survey some methods to study the controllability of nonlinear control systems modeled by partial differential equations, namely:
1. The return method,
2. Power series expansions,
3. Quasistatic deformations.
These methods will be illustrated on quantum systems.
The effect of electronvibrational interactions on the electronic
transport induced by femtosecond omega + 2omega laser fields along
unbiased molecular nanojunctions is investigated. For this, the
photoinduced vibronic dynamics of transpolyacetylene oligomers coupled
to macroscopic metallic leads is followed in a meanfield mixed quantum
classical approximation. A reduced description of the dynamics is
obtained by introducing projective leadmolecule couplings and deriving
an effective Schrödinger equation satisfied by the orbitals in the
molecular region. Two possible rectification mechanisms are identified
and investigated. The first one relies on nearresonance photon
absorption and is shown to be fragile to the ultrafast electronic
decoherence processes introduced by the wire's vibrations. The second
one employs the dynamic Stark effect and is demonstrated to be highly
efficient and robust to electronvibrational interactions.
In this joint work with Gabriel Turinici, we analyse the Lyapunov
trajectory tracking of the Schrödinger equation for a second order
coupling operator. We present a theoretical convergence result; for
situations not covered by the first theorem we propose a numerical
approach and complement it with a second theoretical result.
The control problem of generating unitary transformations is especially relevant to current research in quantum information
processing and computing. Control dynamical landscapes for unitary transformations is analyzed in the inﬁnite dimensional
function space of the timedependent external ﬁeld. The dynamical analysis reveals many essential geometric features of optimal control landscapes for unitary transformations, including bounds on the local landscape slope and curvature. Close examination of the curvatures at the critical points shows that the unitary transformation control landscapes are free of local traps and proper choices of the adaptation matrix may facilitate the search for optimal control ﬁelds producing desired unitary transformations, in particular, in the neighborhood of the global extrema.
A feedback control law is developed for the
stochastic control problems for quantum spin systems.
It is similar to the one we analyzed for the Schroedinger
control system. Also, the time optimal control problem is
discussed for the deterministic quantum spin system.
An algorithm based on the semismooth Newton method is
developed and analyzed. Numerical findings are reported
for the spin half system.
Quantum systems that can be usefully partitioned into a subsystem
interacting with a bath will be considered. For such systems, a
quantumclassical Liouville description of the dynamics is assumed,
while retaining the full quantum equilibrium structure of the system.
The equations of motion may be cast in the form of a nonMarkovian
equation for the diagonal elements of the subsystem density matrix.
The memory kernel in this equation accounts for all coherences in
the system. The conditions under which the memory kernel decays
rapidly as a result of averages over quantum or classical bath
equilibrium structure will discussed. When such decay is rapid, it
will be shown how a lift back to the full phase space results in a
Markovian master equation of motion. This equation leads to a
surfacehopping trajectory description of the dynamics where each
fictitious trajectory accounts for decoherence due to the bath
degrees of freedom. The results will be illustrated by simulations
of nonadiabatic chemical dynamics.
R. Grunwald and R. Kapral, J. Chem. Phys., 126, 114109 (2007).
R. Grunwald, H. Kim and R. Kapral, J. Chem. Phys., 128, 164110 (2008).
An important problem in coherent spectroscopy and quantum information science is to find limits on how close an open quantum dynamical system can be driven to a target state in the presence of dissipation and decoherence. What is the optimal excitation that achieves this objective? We describe these problems in the context of the design of multidimensional NMR experiments that maximize the efficiency of transfer of coherence between coupled spins in the presence of decoherence with the goal of optimizing the sensitivity of these experiments. We present some new mathematical techniques for computing limits on how much coherence or polarization can be transferred between coupled spins in multiple spin topologies.
Joint work with Gil Katz, David Gelman, Mark Ratner and Ronnie Kosloff.
Simulation of many body quantum dynamics scales exponentially
bad with the number of
degrees of freedom. Many methods are devoted to obtain a
restricted many body wavefunction
which still are able to approximate the quantum dynamics. In
the context of system
bath dynamics the surrogate Hamiltonian method the dynamics is
simplified by replacing the
bath Hamiltonian by a simpler version which describes the bath
faithfully up to a specified
time. The computation task becomes even more formidable when
the dynamics takes place
at a finite temperature, then formally the wavefunction has to
be replaced with a density
operator. We present a stochastic methods which allows to
describe finite temperature dynamics
within a wavefunction description. The stochastic methods are
applied for the initial
thermal sampling. In addition the dynamical description of the
bath is extended stochasticly
to take care of dephasing and energy relaxation at long times.
We use this method to
simulate an outstanding problem in coherent control: can we
obtain weak field control of a
branching ratio? The model consists of a ground state and two
excited state potentials. The
target is to control the population in these states using phase
modulation only.
Dynamical decoupling pulse sequences have been used to extend
coherence times in quantum systems ever since the discovery of the
spinecho effect. But while for good reasons the nuclear magnetic resonance (NMR) community has
typically been content with moderate line narrowing, in quantum
computing extremely high levels of coherence are required in order to
perform meaningful computational tasks. In this talk I will describe a method of
recursively concatenated dynamical decoupling pulses, designed to
overcome both decoherence and operational errors [1]. For
boundedstrength, nonMarkovian environments, such as for the
spinbath that arises in electron and nuclearspin based solidstate
quantum computer proposals, it is strictly advantageous to use
concatenated, as opposed to standard periodic dynamical decoupling
pulse sequences. Namely, the concatenated scheme is both
faulttolerant and superpolynomially more efficient, at equal cost
[2,3]. Preliminary experimental results on NMR of 13C in adamantene
(due to Dieter Suter, Dortmund), and NMR of the 31P donor in Si (due
to Steve Lyon, Princeton), demonstrating the advantages of
concatenated decoupling, will also be presented. Time permitting, I
will describe our recent results on the construction of a universal
set of quantum logic gates whose fidelity can be kept arbitrarily high
for essentially arbitrarily long times in the presence of coupling to
a spin bath, by use of concatenated decoupling.
References:
[1] K. Khodjasteh and D.A. Lidar, "FaultTolerant Quantum Dynamical
Decoupling," Phys. Rev. Lett. 95, 180501 (2005).
[2] K. Khodjasteh and D.A. Lidar, "Performance of Deterministic
Dynamical Decoupling Schemes: Concatenated and Periodic Pulse
Sequences," Phys. Rev. A 75, 062310 (2007).
[3] K. Khodjasteh and D.A. Lidar, "Rigorous Bounds on the Performance
of a Hybrid Dynamical DecouplingQuantum Computing Scheme,"
Phys. Rev. A 78, 012355 (2008).
Joint work with Julien Salomon.
In this presentation we present and illustrate a greedy algorithm
that enables in a first stage to design a set of selective laser fields that can in a second stage be used to identify some unknown parameters of quantum systems for a problem of
Hamiltonian Identification.
Joint work with Andrew S. Leathers (Quantum Theory Project, Departments of Chemistry and of Physics,
University of Florida, Gainesville, Florida 32611, U.S.A.).
The interaction of light with a localized (primary) region in a
many atom system undergoing electronic and vibrational transitions
leads to
energy dissipation and
uctuations through both nearly instantaneous
and delayed processes. A fast dissipation typically occurs due
to electronic energy relaxation in the medium, while a delayed
dissipation
arises from vibrational energy relaxation. A theoretical and
computational treatment of these phenomena has been done in terms of
a reduced density matrix (RDM) satisfying a generalized
Liouvillevon Neumann equation.[1] Instantaneous dissipation is described by
a Lindblad term containing electronic transition rates,[2] while the
delayed
dissipation is given by a time integral derived from the
timecorrelation
function (TCF) of atomic displacements in the medium.[3] We
consider cases where the TCF decays exponentially (fast) or as an
inverse
power (slowly). An initial thermal equilibrium can not be
assumed when
there are long lasting interactions between the primary region
and the
medium. We describe a general procedure that provides the
optical response in this case by calculating the difference between
solutions for the RDM with and without excitation by a light pulse. We
present examples for slow relaxation of optical excitation in
CO/Cu(001) and Ag3/Si(111).[4]
1. D. A. Micha, A. Leathers, and B. Thorndyke in "Quantum Dynamics of Complex Molecular Systems" (SpringerVerlag, 2006) D. A. Micha and I.
Burghardt, eds., pp. 165194.
2. D. A. Micha and A. Santana, J. Phys. Chem. A 2003, 107,
7311.
3. A. S. Leathers and D. A. Micha, J. Phys. Chem. A 2006, 110, 749.
4. A. S. Leather, D. A. Micha, and D. S. Kilin, "Density matrix
treatment for an electronically excited adsorbate on a solid
surface", to be published.
Work partly supported by the NSF of the USA, and by the Dreyfus
Foundation.
In the late 1970’s Meyer and Miller (MM) [J. Chem.
Phys.
70,
3214 (1979)] presented a classical Hamiltonian corresponding to
a finite set of electronic states of a molecular system (i.e.,
the various potential energy surfaces and their couplings), so
that classical trajectory simulations could be carried out
treating the nuclear and electronic degrees of freedom (DOF) in
an equivalent dynamical framework (i.e., by classical
mechanics), thereby describing nonadiabatic dynamics in a more
unified manner. Much later Stock and Thoss (ST) [Phys. Rev.
Lett. 78, 578 (1997)] showed that the MM model is actually not
a ‘model’, but rather a ‘representation’ of the
nuclearelectronic system; i.e., were the MMST
nuclearelectronic Hamiltonian taken as a Hamiltonian operator
and used in the Schrödinger equation, the exact (quantum)
nuclearelectronic dynamics would be obtained. In recent years
various initial value representations (IVRs) of semiclassical
(SC) theory have been used with the MMST Hamiltonian to
describe electronically nonadiabatic processes. Of special
interest is the fact that though the classical trajectories
generated by the MMST Hamiltonian (and which are the ‘input’
for an SCIVR treatment) are 'Ehrenfest trajectories', when
they are used within the SCIVR framework the nuclear motion
emerges from regions of nonadiabaticity on one potential
energy surface (PES) or another, and not on an average PES as
in the traditional Ehrenfest model. Examples are presented to
illustrate and (hopefully) illuminate this behavior.
Joint work with Oleksiy Roslyakk
(Chemistry department, University of California Irvine, USA).
Nonlinear optical spectroscopy is commonly formulated
semiclassically, i.e. letting a quantum material interact with
classical fields. The key quantity in this approach is the
nonlinear polarization, characterizing the microscopic response
of the material to the incoming fields. Its calculation can be
based on either the density matrix or the wave function. The
former involves forward propagation in real time and is
represented by double sided Feynman diagrams in Liouville
space, whereas the latter requires forward and backward
propagation in Hilbert space which is carried out on the
SchwingerKeldysh closed time path loop (CTPL). Such loops are
extensively used in quantum field theory of nonequilibrium
states, but doublesided Feynman diagrams have become a
practical tool for the design and analysis of timedomain
nonlinear optical experiments.
Several fundamental ambiguities which arise in the
semiclassical formulation regarding the intuitive
interpretation of optical signals are resolved by combining the
CTPL with a quantum description of the laser fields. In
nonlinear spectroscopy of single molecules, for example, the
signal cannot be given in terms of a classical response
functions as predicted by the semiclassical theory. Heterodyne
detection can be viewed as a stimulated process and does not
require a classical local oscillator. The quantum nature of
the field requires the introduction of superoperator
nonequilibrium Green’s functions (SNGF), which represent both
response and spontaneous fluctuations of the material. This
formalism allows the computation of nonlinear optical processes
involving any combination of classical and quantum optical
modes. Closed correlationfunction expressions are derived for
the combined effects of causal response and noncausal
spontaneous fluctuations. Coherent three wave mixing (sum
frequency generation (SFG) and parametric down conversion
(PDC)) involving one and two quantum optical modes
respectively, are connected to their incoherent counterparts:
twophotoninduced fluorescence (TPIF) and twophotonemitted
fluorescence (TPEF).
We show how twophoton absorption and homodyne detected
difference frequency generation conducted with entangled
photons can be used to manipulate interference effects and
select desired Liouville space pathways of matter. Recently
several groups have applied entangled photon pairs in nonlinear
spectroscopy (near resonance homodyne detected sumfrequency
generation (SFG), two photon induced fluorescence (TPIF) and
twophoton absorption (TPA). It was demonstrated that the
normally quadratic scaling of the signal with the intensity of
the incoming field becomes linear when using entangled photons.
This indicates that the two photons effectively act as a single
particle, interacting with matter within a narrow time window.
This opens new ways for manipulating nonlinear optical signals
and revealing new matter information otherwise erased by
interference.

Processes involving an arbitrary number of classical and
quantum modes of the radiation field are treated within the
same framework.

Loop diagrams can be used to describe all incoherent and
coherent (cooperative) signals.

A unified approach is provided for both resonant and
offresonant measurements. In the latter the material enters as
a parameter in an effective Hamiltonian for the field.

Nonlinear spectroscopy conducted with resonant classical
fields only accesses the causal response function. Quantum
fields reveal the broader SNGF's family which carry additional
information about fluctuations.

Spectroscopy with quantum entangled fields may be described.

"Nonlinear Spectroscopy with Entangled Photons Manipulating
Quantum Pathways of Matter," O. Rosyak, C. Marx and S.
Mukamel, Phys. Rev. A. (In press, 2009).
 "Photon Entanglement Signatures in Homodyne Detected
Difference Frequency Gene," O. Roslyak and S. Mukamel, Optics
Express 17, 1093 (2009).
 "Nonlinear Optical Spectroscopy of Single, Few and Many
Molecules; Nonequilibrium Green’s Function QED Approach," C.A.
Marx, U. Harbola and S. Mukamel, Phys. Rev. A. 77, 022110,
2008.
 "A Unified Description of Sum Frequency Generation,
Parametric Down Conversion and Two Photon Fluoresence," O.
Roslyak, C. Marx and S. Mukamel, Molecular Physics. (In press,
2009).
We develop monotonically convergent algorithms for solving typical quantum optimal control problems in
chemistry and physics. They include (1) statetostate control for a system nonlinearly interacting with a
control and (2) operator pulse design under the influence of dissipation. We discuss the solution
algorithms in a unified manner. As an application of the first algorithm, we consider the
alignment/orientation control of diatomic molecules. The alignment is achieved through the polarizability
coupling between shaped laser pulses and molecules. When the retaining of the aligned state is chosen as a
physical objective, the control pulse is shown to utilize the socalled "coherent destruction of tunneling"
mechanisms. This numerical observation is confirmed by using a simple analytical model.
Second application is associated with (2). In quantum information processing and quantum computer, the
realization of gate operations in physical systems is essential. As the operations should be done with
quite high precision, optimal control approaches could be suitable tools for this purpose. We discuss the
possibility through case studies such as quantum algorithm simulations and suppression of decoherence.
We analyse the possibility of control for a coupled system of
Schrödinger equations on the whole real line for the harmonic
oscillator modeling a single trapped ion. In fact the coupling
is due to the control which acts as a potential and which is
performed by three monochromatic waves which can be switched on
and off, only one of them being active at each time.
By taking the frequency of these waves large enough, we show
that this sytem can be approximated by a much simpler one, the
socalled LawEberly system, for which we can give an explicit
control satisfying all requirements. This enables us to prove
approximate controllability for the original system in the
natural (L^{2})^{2} norms and also in much stronger norms.
This work has been done in collaboration with Sylvain Ervedoza.
Since the development of the laser some 40 years ago, a long
standing dream has been to utilize this special source of
radiation to manipulate dynamical events at the atomic and
molecular scales. Hints that this goal may become a reality
began to emerge in the 1990's, due to a confluence of concepts
and technologies involving (a) control theory, (b) ultrafast
laser sources, (c) laser pulse shaping techniques, and (d) fast
pattern recognition algorithms. These concepts and tools have
resulted in a high speed instrument configuration capable of
adaptively changing the driving laser pulse shapes, approaching
the performance of thousands of independent experiments in a
matter of minutes. Each particular shaped laser pulse acts as
a "Photonic Reagent" much as an ordinary reagent would at the
molecular scale. Although a Photonic Reagent has a fleeting
existence, it can leave a permanent impact. Current
demonstrations have ranged from manipulating simple systems
(atoms) out to the highly complex (biomolecules), and
applications to quantum information sciences are being pursued.
In all cases, the fundamental concept is one of adaptively
manipulating quantum systems. The principles involved will be
discussed, along with the presentation of the state of the
field.
The talk will report on a parametrization of the real symplectic group
in four dimensions. One feature of this parametrization is that it yields
the polar decomposition of a symplectic matrix via the solution of
simple quadratic equations. Applications to the study of squeezing
transformations will be presented. Extensions to higher dimensions will
be discussed.
In this joint work with Mazyar Mirrahimi,
we consider an ensemble of quantum systems described by a density matrix,
solution of a LindbladKossakowski differential equation. We focus on the
special
case where the decoherence is only due to a highly unstable excited
state and where the spontaneously emitted photons are measured by a
photodetector. We propose a systematic method to eliminate the
fast and asymptotically stable dynamics associated to the excited
state in order to obtain another differential equation for the slow
part. We show that this slow differential equation is still of
LindbladKossakowski type, that the decoherence terms and the
measured output depend explicitly on the amplitudes of
quasiresonant applied field, i.e., the control. Beside a rigorous
proof of the slow/fast (adiabatic) reduction based on singular
perturbation theory, we also provide a physical interpretation
of the result in the context of coherence population trapping via
dark states and decoherencefree subspaces. Numerical simulations
illustrate the accuracy of the proposed approximation for a 5level
systems.
Hamiltonian engineering has been shown to be a powerful technique, which can be applied to many different problems that involve steering a quantum system to achieve a desirable outcome, and a particularly promising approach to Hamiltonian engineering is the optimal control approach, i.e., formulating the problem as an optimization problem. However, the problem formulation is important, and although optimization is a wellestablished field, the solution of the resulting optimization problems is usually not trivial, in part because the search space is usually infinite dimensional. To overcome this obstacle the controls must be parametrized, and the parametrization is critical. The most common approach is to approximate the controls using piecewise constant functions. While adequate for some problems, such a parametrization inevitably leads to high bandwidth solutions due to the discontinuities of the fields. We demonstrate that using more natural parameterizations we can significantly reduce the bandwidth of the fields, although at the expense of having to solve more complex optimization problems. Another crucial variable is the problem formulation itself. Often, optimal control problems are formulated using Hamiltonians that incorporate many approximations, e.g., RWA, offresonant excitations and fixed couplings negligible, etc, which inevitably limit what can be achieved by optimal control. We show that we can in principle speed up the implementation of quantum gates several orders of magnitude compared to conventional frequencyselective geometric control pulses for certain systems by avoiding such approximations and taking advantage of the full range of offresonant excitations and couplings available in the optimal control framework. Another problem with Hamiltonian engineering is that the most effective approaches are modelbased, i.e., we require a model of the system, especially its response to external fields, or the functional dependence on the controls. In some cases this isn't a problem and optimal controls can be designed to be robust with regard to model uncertainties. For other problems, however, such as information transfer through spin networks using simple local actuators, it can be shown that the optimal switching sequences are highly modeldependent, while the exact network topology and precise couplings for such systems are usually not known. Such problems call for closed loop optimization. We show that we can effectively solve problems such as finding optimal switching time sequences for such networks by adapting gradientbased optimization algorithms even for problems where the standard evolutionary algorithms fail completely to find acceptable solutions. Finally, there are certain types of problems that Hamiltonian engineering, although an extremely powerful tool for quantum engineering, cannot solve. One such problem is stabilization in the presence of environmental interactions. This problem can in principle be addressed using reservoir engineering. We consider a variant of Markovian reservoir engineering using direct feedback from an indirect measurement such as homodyne detection. We show that if the control and feedback Hamiltonians in this setting are unrestricted and we have some degree of control over the type of measurement we can perform,
then any state can be in principle be stabilized.
In this joint work with Elena Shchepakina we consider a canard trajectory (in the case of scalar slow variable) and a black swan (in the case of vector slow variable) as the result of gluing attractive and repulsive slow integral manifolds, due to the availability of an additional parameter (function in the case of vector slow variable) in the differential system. As a result we obtain the continuous attractive/repulsive slow invariant surface. It is possible to consider the gluing parameter (function) as a special kind of partial feedback control, which guarantees the safety of chemical regimes, even with perturbations, during a chemical process.
In this joint work with Elena Shchepakina we use a geometric singular perturbations method for reducing the model order in chemical kinetics problems. The method relies on the theory of integral manifolds, which essentially replaces the original system by another system on an integral manifold with dimension equal to that of the slow subsystem. Explicit, implicit and parametric representations of a slow invariant manifolds are used.
The powerful techniques of Optimal Control Theory (OCT), used
in recent years to design laser pulse sequences to control
chemical bond breaking, are applied to the problem of laser
cooling in an open system. The result is a striking new
mechanism in which spontaneous emission builds coherences
between all the populated levels creating a pure state, only at
the end of the process transferring the amplitude to the lowest
energy state. This novel mechanism accelerates the cooling
process by exploiting the cooling induced by spontaneous
emission to all the ground electronic state levels, not just
the lowest level. The mechanism suggests the calibration of
cooling in terms of increasing purity of the system as measured
by the quantity Tr(rho2). An analytical theory of the cooling
mechanism is developed in terms of a twostage interplay
between the control fields and the spontaneous emission. One
of the main results of the analytical theory is a differential
equation for the optimal cooling rate. The key components of
the theory – the definition of cooling as purity increase;
the invariance of purity to control fields; and the maximum
rate of approach to absolute zero – correspond to the zeroth,
second and third law of thermodynamics, providing a
thermodynamic framework for laser cooling. The formulation of
cooling in terms of the coherence measure Tr(rho2) has an
additional, interesting implication: that our results carry
over immediately to the problem of control of quantum
decoherence, suggesting both a new mechanism and fundamental
limitations on the control of that process.
Decoherence, which is caused due to the interaction of a quantum system with its environment plagues all quantum systems and leads to the loss of quantum properties that are vital for quantum computation and quantum information processing. In this work we propose a novel strategy using techniques from systems theory to completely eliminate decoherence and also provide conditions under which it can be done so. A novel construction employing an auxiliary system, the bait, which is instrumental to decoupling the system from the environment, is presented. This corresponds to the Internal Model Principle for Quantum Mechanical Systems. Almost all the earlier work on decoherence control employ density matrix and stochastic master equations to analyze the problem. Our approach to decoherence control involves the bilinear input affine model of quantum control system which lends itself to various techniques from classical control theory, but with nontrivial modifications to the quantum regime. The elegance of this approach yields interesting results on open loop decouplability and Decoherence Free Subspaces (DFS). Additionally, the feedback control of decoherence may be related to disturbance decoupling for classical input affine systems, which entails careful application of the methods by avoiding all the quantum mechanical pitfalls. The two concepts are contrasted and an improved theory of disturbance decoupling for general input affine systems is developed. In the process of calculating a suitable feedback the system has to be restructured due to its tensorial nature of interaction with the environment, which is unique to quantum systems. Finally the results are also shown to be superior to the ones obtained via master equations. In order to apply feedback a reliable information extraction scheme using continuous indirect measurements with the help of a quantum probe is outlined. Finally, a methodology to synthesize feedback parameters itself is given, that technology permitting, could be implemented for practical 2qubit systems to perform decoherence free Quantum Computing.
Major League Baseball is a multibillion dollar per year industry that relies heavily on the quality of its schedule. Teams, fans, TV networks, and even political parties (in a way revealed in the talk) rely on the schedule for profits and enjoyment. Only recently have the computational tools of operations research been powerful enough to address the issue of finding "optimal" schedules. Trick will discuss his experiences in scheduling college basketball, major league baseball, and other sports, and show how operations research is revolutionizing the way sports scheduling is done.
The talk will begin with an introduction to the quantum master equation (Liouvillevon Neumann equation), followed by a discussion of how we have used this equation it in a semiclassical algorithm for calculating of nonBornOppenheimer molecular dynamics. I will also discuss the physical origin of decoherence in electronically nonadiabatic molecular dynamics and our method for estimating the decoherence time. The resulting treatment will be validated against accurate quantum dynamics for small molecular systems.
In a topological space, a family of continuous mappings is called universal
if its action, in at least one element of the space, is dense. If the
mappings are unitary or tracepreserving completely positive, the notion of
universality is closely related to the notion of controllability in either
closed or open quantum systems. Quantum controllability in infinite
dimensions is discussed in this setting and minimal generators are found for
full control universal families. Some of the requirements of the operators
needed for control in infinite dimensions follow from the properties of the
infinite unitary group. Hence, a brief discussed of this group and their
appropriate mathematical spaces is also included.
The manipulation and control of quantum systems is fundamental to a host
of emerging
applications from the design of qubits and novel nanoscale devices, to
the control of photochemical reactions as well as atomic and molecular
dynamics. Although there are established techniques to simulate the
evolution of a quantum system, the problem of finding the control
potential which results in a desired evolution is considerably more
challenging. Recent contributions to the development of new quantum
control methodologies and optimal control formulation are discussed. In
particular, the investigation of theoretical issues such as the
appropriate choice of function spaces for the control and the nonconvex
structure of the optimization problems as well as the interplay between
discretization and optimization are considered. Accurate and
computationally efficient algorithms for computing the optimal controls
which take advantage of the underlying physics are introduced with a
focus on KrylovNewton methods for solving controls for fast state
transitions in a system.
We have recently developed a hierarchical equationsofmotion
(HEOM) approach to nonperturbative and nonMarkovian quantum
dissipation. It is a unified and exact theory for arbitrary
coupling Gaussian environments of distinct nature: bosonic
versus fermionic, and canonical versus grand canonical
ensembles. It admits also an arbitrary timedependent external
field driving. Two systems will be used to elaborate both the
formulation and implementation aspects of the theory.
In an electron transfer (ET) system, the bath environment
serves as a canonical bosonic ensemble, responsible for the
system decoherence and energy relaxation. The validation of
Zusman equation will be discussed, on the basis of exact HEOM
results.
In a quantum transport setup, a molecule or quantum dot is
placed in contact with electrodes under applied voltage. Each
electrode reservoir serves as a grand canonical fermion
ensemble. It is responsible not only for decoherence and energy
relaxation, but also for the fermion particle (i.e., electron)
transport in/out of the system. The HEOMbased quantum
transport theory will be summarized, together with the
calculated transient currents through model quantum dot systems
and the current spectrums in response to various forms of
external timedependent applied voltage.
Support from RGC of Hong Kong Government is acknowledged.
R.X. Xu and Y. J. Yan, Phys. Rev. E, 75, 031107 (2007).
J. S. Jin, X. Zheng, and Y. J. Yan, J. Chem. Phys. 128, 234703
(2008).
X. Zheng, J. S. Jin, and Y. J. Yan, J. Chem. Phys. 129, 184112
(2008).
X. Zheng, J. S. Jin, and Y. J. Yan, New J. Phys. 10, 093016
(2008).
Joint work with Jinshuang Jin.
We present a hierarchical equationsofmotion (HEOM) formalism of quantum dissipation theory
[J. Chem. Phys. 128, 234703 (2008)], which is formally exact, practically tractable, and numerically
convergent. It characterizes the transient current transport dynamics of arbitrary dissipative manyelectron
systems, in contact with electrodes under arbitrary temperatures and external fields. The HEOM
approach provides a useful theoretical tool to study various transient and stationary properties of
manybody systems far away from equilibrium. With an efficient hybrid scheme accounting for the bath
correlation functions, we demonstrate accurate transient response current driven by timedependent
applied voltages in both sequential and cotunneling regimes.
In this lecture we shall present a survey of recent work on several topics related with numerical approximation of waves.
Control Theory is by now and old subject, ubiquitous in many areas of Science and Technology. There is a quite wellestablished finitedimensional theory and many progresses have been done also in the context of PDE (Partial Differential Equations). But gluing these two pieces together is often a hard task from a mathematical point of view.
This is not a merely mathematical problem since it affects modelling and computational issues. In particular, the following two questions arise: Are finitedimensional and infinitedimensional models equally efficient from a control theoretical point of view? Are controls built for finitedimensional numerical schemes efficient at the continuous level?
In this talk we shall briefly analyze these issues for the wave equation as a model example of propagation without damping. We shall show that high frequency spurious oscillations may produce the divergence of the most natural numerical schemes. This confirms the fact that finite and infinitedimensional modelling may give completely different results from the point of view of control. We shall then discuss some remedies like filtering of high frequencies, multigrid techniques and numerical viscosity.
Similar questions arise when building numerical approximation schemes for nonlinear Schrödinger equations or in other contexts as when designing, for instance, absorving boundary conditions or developping the method of Perfectly Matched Layers (PML) for the wave equation.